Bipolar transistors exhibit the characteristics of a switch when driven to the extreme conditions of cut-off and saturation mode. The advantages of driving a transistor into saturation mode include a low voltage across collector and emitter inputs known as a saturation voltage (V.sub.CESAT), and a low collector-emitter resistance, which provides for optimum efficiency. As the transistor is driven still further into saturation mode or into deep saturation, the saturation voltage increases with collector current (I.sub.C), due mainly to collector resistance, and efficiency decreases due to the resulting power dissipation which is a function of a collector-emitter voltage (V.sub.CE) times I.sub.C.
In addition to the decrease in efficiency, a bipolar transistor driven into deep saturation causes minority carriers from its collector to be injected into its base. As a result, the turn off time of the transistor is adversely affected and becomes a function of the transistor's storage time (t.sub.s) since the minority carriers must be swept out of the base before the transistor is turned off. Switching speed of bipolar transistors is a major advantage and numerous techniques have been employed to keep transistors out of saturation mode and/or deep saturation mode for improved turn off times.
A common antisaturation technique comprises shunting a collector-base junction of a NPN transistor with a Schottky diode. In order for a transistor to operate in saturation mode, both the base-emitter and collector-base junctions must be forward biased. The Schottky diode exhibits a lower forward voltage (V.sub.F) than the forward biased collector-base junction and thus clamps the junction at the lower voltage, therein preventing the transistor from operating in the saturation mode. Excess base current is fed to the collector of the transistor by the Schottky diode which decreases the effective efficiency of the transistor. The V.sub.F of the Schottky diode changes with temperature at a rate equal to approximately one half the rate of change of the transistor's collector-base voltage with temperature. This method, therefore, losses its effectiveness at high temperatures. An antisaturation technique similar to using the Schottky diode but improving on the temperature tracking problem is described in Eshbaugh, U.S. Pat. No. 4,675,548. Here the collector-base junction of the transistor which is to be kept from operating in the saturation mode is shunted by a collector-emitter junction of a second transistor to limit the collector-base voltage of the saturation protected transistor. Like the Schottky diode solution, this technique steers excess base current of the protected transistor to its collector and is not suitable for many analog applications.
Another antisaturation technique using a parasitic lateral PNP transistor is taught in Yoshimura, U.S. Pat. No. 4,021,687. The lateral PNP transistor turns on before the saturation protected NPN transistor goes into the deep saturation mode and steers excess base current through a third transistor. This technique, while preventing the transistor from going into the deep saturation mode, still allows the protected transistor to go into the saturation mode and further steals base current from the protected transistor which may adversely affect its input impedance. Thus what is needed is an antisaturation circuit useful in analog applications, which senses a protected transistor's collector-base voltage to keep it out of saturation by modifying its collector current accordingly without stealing its base current and thus changing the input impedance of the device.